In recent years, use of higher integration degree and finer processes as well as lower process temperature in the device processes are accelerated. Therefore, in regard to a silicon wafer, there are required integrity of device-active region of the surface layer and an improvement of gettering ability for capturing impurities of metals and so forth formed due to increase of bulk micro defects (BMD) consisting of oxide precipitates (nuclei) in the bulk, and so forth.
As for these requirements, various approaches have been attempted. For example, nitrogen doping is performed during the growth of a silicon single crystal by the Czochralski method (CZ method), thereby a silicon single crystal ingot in which growth of grown-in defects is suppressed and oxygen precipitation is promoted can be produced. Then, a mirror surface wafer sliced from such a nitrogen-doped silicon single crystal and polished is subjected to a high temperature annealing in which the wafer is subjected to a heat treatment at a high temperature (1100° C. to 1350° C.) for a long period of time using argon gas, hydrogen gas or the like, and thereby a wafer (annealed wafer) simultaneously realizing integrity of the surface layer and increase of density of oxygen precipitation nuclei in the bulk can be produced.
When a silicon single crystal doped with nitrogen is grown by the CZ method, nitrogen is taken in the crystal from silicon melt while it undergoes segregation. At this time, since the segregation coefficient of nitrogen is extremely small, i.e., 0.0007, nitrogen concentration in the single crystal ingot becomes low near the shoulder and becomes high near the tail. The ratio between the nitrogen concentrations is around 3 to 7 times. Therefore, the sizes of oxygen precipitation nuclei existing in the crystal also vary depending on the positions for slicing. Thus, a wafer sliced from the ingot at a position near the shoulder, which has a low nitrogen concentration, shows small size of oxygen precipitation nuclei, and a wafer sliced from the ingot at a position near the tail, which has a high nitrogen concentration, contains oxygen precipitation nuclei having a relatively large size.
Further, when such a high temperature heat treatment as the aforementioned argon annealing is applied to a mirror surface wafer, although oxygen precipitation nuclei having a relatively large size remain even after the heat treatment, oxygen precipitation nuclei having a small size are melted. Therefore, if BMD densities of annealed wafers sliced from one nitrogen-doped silicon single crystal at various positions are compared, variation of the density was observed depending on the positions at which the single crystal ingot was sliced.
Such variation of BMD density among nitrogen-doped annealed wafers as described above also results in variation of the gettering ability of the wafers, and this constitutes one of the causes of reduction of yield and productivity of devices as a result.